rbd: move to obj_req->img_extents

In preparation for rbd "fancy" striping, replace obj_req->img_offset
with obj_req->img_extents.  A single starting offset isn't sufficient
because we want only one OSD request per object and will merge adjacent
object extents in ceph_file_to_extents().  The final object extent may
map into multiple different byte ranges in the image.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>